Polypropylene resins have been extensively used in automotive applications and in fields such as electrical appliances, miscellaneous goods, films or the like, because the articles molded therefrom are excellent in rigidity and high-temperature rigidity. The resin compositions for interior automotive trim parts have been required to be excellent in not only rigidity and high-temperature rigidity, but also impact resistance of molded articles and flowability of the resin compositions on molding. However, conventional polypropylene resins have been limited in the use range, since the articles molded therefrom are deficient in impact resistance.
As typical polypropylene resins having improved impact resistance are known propylene/ethylene block copolymers wherein propylene and ethylene are block copolymerized and polypropylene resin compositions wherein polypropylene is incorporated with an ethylene/.alpha.-olefin copolymer rubber. Such polypropylene resins have lower rigidity than a propylene homopolymer. If those resins are incorporated with inorganic fillers such as talc, as the occasion demands, to make up for lowered rigidity, they are used mainly as molding materials for exterior automotive trim parts including automotive bumpers and interior automotive trim parts including instrument panels, console boxes, resin pillars and trims. However, the incorporation of inorganic fillers in a large amount is required to enhance the rigidity of the resin composition, which results in lowering the flowability of the resultant composition and also in increasing the specific gravity of the resultant molded articles, with an increased weight of the products.
With a view to improving the impact resistance and stress-whitening resistance of .alpha.-olefin polymers such as polypropylene and polyethylene and .alpha.-olefin copolymers such as ethylene/propylene copolymer, various resin compositions have been proposed wherein hydrogenated diene copolymers are incorporated therein.
For example, JP-B-59-37294 discloses a resin composition wherein a hydrogenated 1,2-polybutadiene is incorporated in polymers or copolymers of .alpha.-olefin with 2 to 20 carbons, especially ethylene homopolyhmer, propylene homopolymer or ethylene/propylene copolymer. JP-B-62-45883 and JP-A-4-342752 disclose a resin composition wherein a hydrogenated diblock copolymer of polybutadienes consisting of 1,2-block and 1,4-block is incorporated in .alpha.-olefin polymers or copolymers, in particular polypropylene.
JP-A-5-132606 discloses a resin composition wherein a hydrogenated product of polybutadiene/conjugated diene block copolymers is incorporated in a crystalline propylene/ethylene block copolymer. JP-A-1-168743 and JP-A-1-168744 disclose a resin composition wherein a hydrogenated product of isoprene/butadiene copolymers is incorporated in polyolefins.
The resin compositions are improved in both stress-whitening resistance and impact resistance, but remarkably lowered in rigidity and high-temperature rigidity. Thus a practical use of such resin compositions is limited to the field of soft materials.
To enhance the reduced rigidity and high-temperature rigidity in the above-described resin compositions wherein the hydrogenated diene block copolymers are incorporated in the propylene/ethylene block copolymers, it is created that the rigidity and high-temperature rigidity of the molded articles are improved with no increase in the specific gravity, by substitution for said propylene/ethylene block copolymers of high stereoregular polypropylenes disclosed in JP-A-62-187707. However, a reduction in impact resistance of the molded articles is anticipated. As mentioned above, the impact resistane of the polypropylene resin comositions is improved by the incorporation of the ethylene/.alpha.-olefin copolymer rubber and the hydrogenated polybutadiene polymer and copolymer. However, the incorporation of these rubber components in a large amount brings about new issue of lowering the rigidity and high-temperature rigidity of the molded articles as well as the flowability of the resin compositions. Thus, it has been desired to develop a polypropylene resin composition being capable of improving the impact resistance of the molded articles with no reduction in the rigidity and high-temperature rigidity of the molded articles as well as the flowability of the resin compositions.
Accordingly, an object of the present invention is to provide a polypropylene resin composition having a well-balanced combination of the flowability of the resin composition and the rigidity, high-temperature rigidity and impact resistance of the molded article, which is suitable as a molding material for interior automotive trim parts.